1. Field of the Invention
The present invention relates to a recording medium, and particularly to a recording medium suitable for use in ink-jet recording systems, a production process thereof, and an image forming process using such a recording medium.
2. Related Background Art
An ink-jet recording system is a system that minute droplets of an ink are ejected by any one of various working principles to apply them to a recording medium such as paper, thereby making a record of images, characters and/or the like, and is being quickly spread in various applications because it has features that recording can be conducted at high speed and with a low noise, color images can be formed with ease, recording patterns are very flexible, and development and fixing process are unnecessary.
Further, it begins to be applied to a field of recording of full-color images because images formed by a multi-color ink-jet system are comparable in quality with multi-color prints by a plate making system and photoprints by a color photographic system, and such printed images can be obtained at lower cost than the usual multi-color prints and photoprints when the number of copies is small.
With the improvement in recordability such as speeding up and high definition of recording, and full-coloring of images in the ink-jet recording system, recording apparatus and recording methods have been improved, and recording media have also been required to have higher properties.
In order to enhance absorbency, coloring ability and resolution, there have been proposed a wide variety of recording media obtained by forming a porous layer with inorganic particles, for example, recording media, in which a porous layer comprising an alumina hydrate is provided on a substrate as described in Japanese Patent Application Laid-Open No. 2-276670.
As described in Japanese Patent Application Laid-Open No. 4-101880, recording media, in which a transparent ink-fixing layer formed of a resin capable of being dissolved in or swelled by a solvent contained in an ink is provided on a substrate, have also been proposed.
However, inks of the type that a dye component is dissolved in a solvent have heretofore been often used in the conventional ink-jet recording system.
When such a dye ink is used, the resulting print is naturally poor in light fastness and ozone fastness and hence Involves a problem that it undergoes fading or color change when it is stored for a long period of time. Therefore, as described in Japanese Patent Application Laid-Open No. 58-136482 and U.S. Pat. No. 5,374,475, there have been proposed recording media in which a porous layer comprising a thermoplastic polymeric material is provided on a substrate, and the porous layer is melted by the action of heat and pressure after printing to make it dense.
As described in Japanese Patent Publication No. 2-31673, there have also been proposed recording media having an ink-receiving layer of a two-layer structure that an inorganic pigment layer absorbing an ink in a great amount is formed on a substrate, and an ink-receiving layer comprising a thermoplastic polymer is provided as the outermost layer.
Japanese Patent Application Laid-Open No. 9-296067 has proposed recording media having an ink-receiving layer in which a great number of interstices are formed as pores among polymer particles.
However, the above-described prior art documents have involved the following problems. Namely, in the conventional recording media, the so-called slurry-like plastic pigment, which is prepared by subjecting at least one vinyl monomer to emulsion polymerization, or the like is suitably used. The fine particles generally used in the above-described Japanese Patent Application Laid-Open No. 58-136482, U.S. Pat. No. 5,374,475 and Japanese Patent Publication No. 2-31673 are spherical and uniformly arranged as illustrated in FIG. 6, and the ink absorbency of the recording media has been imparted by the interparticle pores.
A porous resin layer having pores formed among such spherical particles has an effect to absorb an ink impacted on the recording medium after ejection of the ink and make the diameter of dots formed uniform. As a result, a certain degree of resolution has been able to be achieved in an image formed therefrom.
When an amount of the ink to be ejected is increased upon printing for the purpose of increasing a printing speed, however, a solvent (water) in the ink cannot be fully absorbed in the pores formed in the recording medium, so that problems such as ink running and bleeding have arisen.
Therefore, it has been attempted to enhance ink absorbency by containing porous inorganic particles in a porous resin layer. In this case, it is difficult to control the mixing ratio of the thermoplastic resin particles to the porous inorganic particles contained therein. When the content of the porous inorganic particles is high, the amount of the ink absorbed is increased. However, the sufficient content lowers the strength of the porous resin layer, so that such problems that separation or defects are incurred in handling have been encountered upon the production of such a recording medium or formation of an image on the resulting recording medium.
According to the recording media described in Japanese Patent Application Laid-Open No. 9-296067, polymer particles in an emulsion are aggregated by instabilizing the emulsion before, upon the formation of an emulsion-containing layer, all water in the emulsion is vaporized out of the layer, the resultant layer is then dried, thereby causing interstices among the particles to remain as pores to provide a porous resin layer.
However, this process has involved a problem that since the aggregation is conducted by using a gelling agent before the drying, it is impossible to stably control the aggregation, so that-the degree of aggregation becomes varied, and as illustrated in FIG. 7, the size and number of the pores become uneven to partially produce portions low in absorbency and also to specifically produce large pore portions, whereby absorption of an ink is varied to make an optical density of image unstable.
With the foregoing problems in view, it is an object of the present invention to provide a recording medium which has good ink absorbency and little variation in the absorption of an ink and can provide clear and bright images high and even in optical density, and an image forming process using such a recording medium.
The above object can be achieved by the present invention described below.
According to the present invention, there is thus provided a recording medium comprising a substrate and at least one porous resin layer formed on the substrate, wherein the porous resin layer comprises heteromorphic microspheres.
According to the present invention, there is also provided a process for producing a recording medium having at least one porous resin layer on a substrate, which comprises the steps of applying a coating formulation comprising polymer colloid, in which heteromorphic microspheres are dispersed, to the Substrate, and drying the coating formulation at a temperature lower than the lowest film-forming temperature of the heteromorphic microspheres, thereby forming the porous resin layer.
According to the present invention, there is further provided an image forming process, comprising the step of ejecting an ink by an ink-jet recording system on the porous resin layer of the recording medium described above to conduct recording, and then heating the recording medium at a temperature not lower than the lowest film-forming temperature of the heteromorphic microspheres, from which the porous resin layer is formed, thereby forming the porous resin layer into a film.
According to the present invention, there is still further provided an image forming process, comprising the step of ejecting an ink by an ink-jet recording system on the porous resin layer of the recording medium described above to conduct recording, laminating the recording medium on a transfer-printing medium with the porous resin layer opposed to the transfer-printing medium, and then heating and pressing the recording medium at a temperature not lower than the lowest film-forming temperature of the heteromorphic microspheres, from which the porous resin layer is formed, from the side of the substrate of the recording medium, thereby transferring the porous resin layer to the transfer-printing medium.
According to the present invention, even and bulky pores can be stably formed in an ink-absorbing layer by arranging and bonding the resin particles in the form of a heteromorphic microsphere, thereby providing recording media which have high ink absorbency upon absorption of an ink and permit the formation of images having high and even optical density.
According to the present invention, there can be provided recording media which permits the provision of images having excellent water fastness and weather fastness and withstanding long-term storage, have a strong film and are easy to handle, and an image forming process which can actually achieve excellent image quality and permits the formation of an image by transfer printing
The present invention has been led to completion by investigating the findings obtained by tests by the present inventors in further detail.